This invention relates in general to induction heating of elongated metal workpieces and, more particularly, to a method and apparatus for inductively heating to a uniform surface temperature an elongated workpiece having an extended intermediate portion of uniform cross-section and relatively short, axially spaced, end or other portions of enlarged cross-section.
The invention is particularly applicable to the heating of so-called sucker rods which are employed in oil wells and the like and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be used for inductively heating a variety of elongated workpieces having an intermediate portion of substantially uniform cross-sectional shape extending between axially spaced portions of enlarged cross-sectional shape.
Sucker rods are utilized in the petroleum industry as a connecting link between a down hole oil well pump and the lifting or pumping device on the surface. Each rod is quite long, normally being of the order of magnitude of 25-30 feet in length. A major portion of the length of the rod, hereinafter referred to as the intermediate or central bar portion of the rod, has a uniform cross-section which may be of circular, hexagonal, or the like cross-section and is generally uniform over its entire length. At their opposite ends, the rods are provided with comparatively short enlarged or upset end portions for facilitating interconnection of a plurality of the rods together in end-to-end relationship with each other. While there are a number of specific or detailed modifications which may be included in the sucker rod configurations of different manufacturers, almost all such rods have the foregoing general conformation and characteristics.
As one of their manufacturing steps, the sucker rods are usually heated to a given predetermined surface temperature and then passed through an electrostatic spray chamber for application of a coating of paint or plastic-like material thereto. Since the rods need only be surface heated for this purpose, induction type heating finds particular use in this environment. However, because sucker rods do not have uniform cross-sections throughout the entirety of their length, it is necessary to in some way compensate for the heating variations which otherwise occur at the enlarged or upset end portions. Unless each rod is heated to a uniform surface temperature over the entire length thereof, the paint of plastic-like coating subsequently applied to the rods will not have a uniform consistency, thickness, etc. thereon, particularly as between the elongated central bar portions and the enlarged ends.
A number of different arrangements to compensate for this difficulty have been previously proposed and employed heretofore. Such solutions include voltage regulation for the inductor, delay on or off timers, various alternative inductor configurations and the like. However, none of these proposed or previously employed arrangements have satisfactorily solved the problem of uniformly heating the surface of sucker rods or other similar elongated workpieces having sections of enlarged cross-section at their opposite ends.
It has been considered desirable, therefore, to devise an improved induction heating arrangement for sucker rods or similar workpieces which facilitates induction heating thereof to a substantially uniform surface temperature throughout the entire length of the rods. The method and apparatus described herein is deemed to provide such an improved heating arrangement which satisfactorily overcomes the foregoing problems and others.